The present invention relates to a ceramic coating bonded to a metal member for use in exhaust equipment of internal combustion engines, etc. and a method of producing it.
For metal members such as exhaust equipment of internal combustion engines, etc., which are exposed to corrosive gases at high temperature and severe heat shock, it was proposed to form ceramic linings on the inner surfaces of such metal members to impart a heat resistance, a corrosion resistance and a heat shock resistance.
Significant problems with such ceramic coatings are that since they are subjected to severe heat shock by a high-temperature exhaust gas, a large stress is generated on the boundaries between the ceramic coatings and the metal members due to the difference in thermal expansion between them, leading to the peeling of the ceramic coatings from the metal members. Also since the ceramic coatings have much smaller heat conductivity than the metal, an extremely large temperature gradient appears in the ceramic coatings, thereby generating a large stress in the ceramic coatings, which leads to the peeling and cracking of the ceramic coatings.
In general, although ceramics have large compression strength, they have little tensile strength and are extremely brittle. Accordingly, they are extremely less resilient to thermal shock.
To solve these problems, various proposals were made.
For instance, Japanese Patent Laid-Open No. 58-51214 discloses exhaust gas equipment for internal combustion engines comprising a metal equipment body to be exposed to a high-temperature exhaust gas, an inner surface of which is coated with a refractory layer composed of a mixture of refractory material particles and a heat-resistant inorganic binder.
In addition, as a method of forming a ceramic layer by attaching ceramic particles after applying an inorganic binder to an inner surface of a metal member, Japanese Patent Laid-Open No. 58-99180 discloses a method of producing exhaust gas equipment for internal combustion engines which comprises the steps of forming a heat-resistant layer by coating an inner surface of a metal equipment body to be exposed to a high-temperature exhaust gas with a slip composed of a mixture of refractory material particles, an inorganic binder and frit; forming a refractory, heat-insulating layer by coating the heat-resistant layer while it is in a wet state, with refractory, heat-insulating particles; and then, after solidifying the heat-insulating layer, forming a heat-resistant layer thereon by coating the refractory, heat-insulating layer with a slip composed of a mixture of refractory material particles, an inorganic binder and a frit. If necessary, the heat-resistant layer can be coated with a further refractory, heat-insulating layer, and a further heat-resistant layer repeatedly to produce a ceramic coating.
However, these methods fail to provide sufficient bonding strength between the ceramic layer and the metal, leaving the problem that ceramic layers are likely to peel off from the metal members along the bonding boundaries or in the ceramic layers themselves by heat shock. Thus, they are not satisfactory in durability for a long period of time.
Recently, ceramic paints and coating materials containing metal alkoxides as binders were developed. However, these materials are extremely expensive, and it is difficult to coat them in sufficient thickness to enable them to endure use for a long period of time.
Further, Japanese Patent Laid-Open No. 59-12116 discloses a composite ceramic material comprising inorganic hollow particles dispersed in a ceramic matrix. However, mere dispersion of inorganic hollow particles in a matrix fails to provide a coating having good bonding strength to a metal surface and high heat shock resistance, though it has sufficient heat resistance. In addition, since the inorganic hollow particles have small strength, they are easily broken, leading to peeling and cracking of the resulting ceramic coating.
Next, it has been found that when a ceramic coating bonded to a metal member is exposed to a corrosive exhaust gas, etc. at high temperature for a long period of time, the corrosive exhaust gas penetrates into the ceramic layer and reaches to the boundary with the metal, thereby oxidizing the metal surface. The oxidation of the metal surface leads to extreme decrease in bonding strength between the ceramic layer and the metal layer, which means that the ceramic layer is easily peeled off by mechanical shock or heat shock.